The invention relates to a method and apparatus for controlling an operational mixture of fuel and air supplied to an internal combustion engine, and, more particularly, to a method and apparatus for regulating the fuel-air ratio of the mixture in accordance with the oxygen content of the engine exhaust gases.
In a known method and apparatus for regulating the fuel-air ratio of an operational mixture for an internal combustion engine, such as that disclosed in U.S. Pat. No. 4,208,993, issued June 24, 1980 to Peter, a .lambda. sensor is connected with a regulating apparatus for influencing the fuel-air ratio. The .lambda. sensor has a temperature-dependent internal resistance which influences the operational readiness of the .lambda. sensor. In order to ascertain the operational readiness of the .lambda. sensor, a reference voltage is supplied through a resistor to an output of the .lambda. sensor to oppose the voltage signal generated by the .lambda. sensor, and the resultant voltage at the .lambda. sensor output is examined as to a minimum jump indicating the operational readiness of the .lambda. sensor by two comparison devices having different threshold voltages. The output signals of the two comparison devices are logically linked and the signal resulting therefrom is evaluated as a standard for the operational readiness of the .lambda. sensor by an evaluation circuit which generates a readiness signal or an unreadiness signal to enable or disenable a first functional mode of the regulating apparatus wherein the regulating apparatus is controlled by the .lambda. sensor.
One of the comparison devices serves to ascertain whether the sensor signal is higher or lower than the reference voltage which determines the regulating point and which is located within the voltage jump of the .lambda. sensor output signal when .lambda.=1. The regulating device is controlled by the output signal of the comparator. A desired regulating point or a desired .lambda. can be established with the aid of the regulating device when the reference voltage is located within the .lambda. sensor voltage jump at .lambda.=1. This arrangement has the disadvantage, however, that the effective switchover point of the comparator relative to the internal voltage of the .lambda. sensor shifts in accordance with temperature; accordingly, because of the finite steepness of the .lambda. sensor voltage jump at .lambda.=1, the result is a temperature-dependent deviation from the desired control value of the control value actually generated at the output of the comparator.